GUIDELINE –
LASERWELDING OF
PLASTICS
Evosys Laser GmbH, Henri-Dunant-Straße 8, 91058 Erlangen
Content
Laserbeam
1. Foreword
laser-transparent
2. Procedural principle
3. The process variants
3.1 Contour welding
3.2 Quasi-simultaneous welding
5.3 Simultaneous welding
4. Materials
4.1 Material selection
4.2 Molecular order and transmission
4.3 Additives or functional additives
4.4 Fillers
4.5 Reinforcers
4.6 Coloring
laser-absorbing
Figure: Principle of laser transmission welding
This absorbs the laser radiation on the surface and
is melted. The heating and melting of the
transparent joining partner is largely done by
thermal conduction from the absorbent to the
transparent joining partner. The weld seam is
produced due to the relative movement between the
laser beam and the component to be welded.
5. Design layout of the joining zone
5.1 Process variant-independent criteria
5.2 Process variant-specific criteria
6. Checklist
1. Foreword
The information listed below about the design of
plastic moldings for laser welding is based on our
long-standing experience. We have tried through
the proper selection of the most important issues to
ensure a brief overview and quick introduction to the
technology. On your request, the EVOSYS
application experts are happy to support you
personally in the design of your assembly.
2. Procedural principle
When laser transmission welding, the two parts to
be joined are placed in the lap joint and affixed in
this position already before welding by means of a
suitable clamping device. The laser beam passes
through one of the two joining partners, which is
largely transparent to the laser radiation, through to
the second joining partner (Figure 1).
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3. The process variants
Depending on the relative movement between the
laser beam and joining assembly as well as the type
of the energy input, the laser welding of plastics is
divided into different process variants.
3.1 Contour welding
In contour welding, the weld seam is traversed once
or several times with a focused laser beam. When
passing over a portion of the weld seam by the laser
beam, this is heated up to the melting temperature
range and then immediately re-solidifies so that a
fusion of the two joining partners immediately
results.
Contour welding is always to be used when melt
flash is to be avoided for aesthetic or functional
reasons. In addition, you are only limited with
respect to the component dimensions by the
traversing range of the kinematics used in contour
welding so that even components with dimensions
> 1 m can be processed.
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the clamping technology, the two components to be
joined move toward one another and the melted
mass is laterally expelled at the weld seam. This
collapse serves to compensate for component
unevenness and can be used simultaneously for
process control.
5.3 Simultaneous welding
With simultaneous welding, the entire weld seam is
simultaneously heated and melted by one or more
laser sources. The arrangement of the beam
sources and beam formation by means of suitable
optical elements must be carried out so that the
energy input per unit length introduced during the
welding process is as even as possible over the
entire weld seam.
Figure: Principle of contour welding
Contour welding also offers advantages over other
welding processes if a temperature control is to be
realized through pyrometer.
Furthermore, contour welding is used for radially
symmetrical components where the laser beam
must meet the component radially, because of the
joint geometry. In such cases it makes sense to
either move the component by means of a rotational
axis and to keep the laser stationary or to move the
laser by means of a suitable optical configuration
around the component.
3.2 Quasi-simultaneous welding
With quasi-simultaneous welding, the weld seam is
repeatedly traversed with a focused laser beam at
a high speed. All areas of the weld seam are heated
and melted at nearly the same time (i.e. quasisimultaneously).
Figure: Principle of quasi-simultaneous welding
Through the simultaneous melting of the entire
welding contour and the clamping force applied by
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Figure: Principle of simultaneous welding
Due to the simultaneous melting of the entire
welding contour and the force applied by the
clamping technology, the two components to be
joined move towards each other just like with quasisimultaneous welding and the melted mass is
laterally expelled at the weld seam. The collapse
serves on the one hand to compensate for
component unevenness and on the other hand it
can be simultaneously used for process monitoring.
A weld seam produced in the simultaneous welding
process is basically the same in appearance as a
weld seam produced in quasi-simultaneous welding
processes.
4. Materials
4.1 Material selection
Thermoplastics take a special position among
polymer materials due to the molecular structure
and the associated thermal properties. They are
both fusible and malleable. This behavior allows the
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welding of two thermoplastic polymers. The
prerequisite for this is a chemical and thermal
minimum compatibility of the two materials. In
addition, the melting ranges of the two materials to
be welded must overlap. This is the case especially
when connecting similar thermoplastics. The
selection of the individual polymers in mixedmaterial welding is much more difficult and usually
requires intensive inspections. Here we strongly
recommend getting in touch with our experienced
EVOSYS application experts.
4.2 Molecular order and transmission
Amorphous thermoplastics only absorb a small
portion of the incident laser radiation at specific
wavelengths. In contrast, the optical properties of
semi-crystalline thermoplastics differ significantly
from those of the amorphous thermoplastics. The
crystalline superstructures present there cause
multiple undirected reflection of the laser radiation
(scattering). The absorption coefficient rises with an
increasing degree of crystallization. The reflection
of infrared radiation by the material itself is larger
with semi-crystalline thermoplastics than with
amorphous thermoplastics.
Figure: Radiation portions for the material passage
(beam direction from left to right)
The wavelengths usually used for the laser welding
are also partly absorbed (A), reflected (R) and
scattered (T) in an uncolored and unfilled plastic.
Figure: Overview of weldability of different materials
Plastics are rarely put to use in their pure form.
Mostly mechanical and thermal as well as optical
adjustments of the polymer are made.
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In addition to the possibilities of chemical
modification of the monomers, additives, fillers or
reinforcing materials are often used today.
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4.3 Additives or functional additives
Additives are mixed with the polymer in small
amounts and thus cause a change to the
appropriate properties. As the additives are usually
very different in size, shape and color compared to
the basic material, in addition to the desired
properties, other properties of the plastic can also
be changed. The most commonly used additives
are listed in the following:




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Flame retardant
Solid lubricants
Nucleating agent
Metals
Plasticizers
Due to the high variety of different additives on the
market, we recommend consulting an EVOSYS
application expert in any case regarding influencing
the behavior of the laser radiation welding.
4.4 Fillers
The term 'fillers' is generally used for powder,
organic or inorganic substances. Their addition to
the base polymer is usually intended to increase the
volume and weight as well as to improve the
technical usability in terms of strength, elongation
and elasticity.
Polymers modified with organic fillers can usually
not be processed with the joining method of laser
welding due to the very strong absorbance of
additives and the low decomposition temperatures
of the organic particles.
Much more relevant for the laser welding of plastic
materials is the use of inorganic fillers (calcium
carbonate, talc, glass beads). However, as a
number of different versions also exist on the
market here and the filler proportion has a huge
impact, the exact impact for the specific application
case should be metrologically clarified in advance.
4.5 Reinforcing materials
Another technically important method for modifying
the mechanical properties of a plastic is the use of
reinforcing materials. Reinforcing materials are
generally understood as filler particles that serve to
increase the mechanical strength. Characteristic is
the fiber-like geometry with a large length to
diameter ratio. Today, synthetic inorganic
substances, such as glass, carbon and aramid
fibers, are used for polymer reinforcement.
Glass fibers are used these days for most technical
applications with high mechanical loads. The impact
on laser radiation welding lies on the one hand at
varying transmission properties with varying fiber
content. On the other hand, the influence on the
seam strengths must be considered. Despite the
high transparency of glass in a natural state in any
form, the addition of glass fibers significantly affects
the transmission properties of thermoplastic
materials. Nevertheless, polymers with a fiber
content of up to 50% can be welded by laser
radiation with suitable process control.
It is possible to use carbon fibers for applications in
which the stiffness and strength of glass fibers are
insufficient. Since C-fibers are made of graphite,
these types of reinforced materials have an
extremely high absorption of radiation in the near
infrared range. The absorption properties of carbon
fiber-reinforced polymers are very similar to those
of carbon black filled polymers. The use in laser
plastic welding applications is only possible as
absorbing joining parts due to this property.
Specifically, consultation an EVOSYS application
expert will provide quick information.
Specifically, consultation with an EVOSYS
application expert will provide quick information.
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4.6 Coloring
Figure: Degree of difficulty of color combinations
Seldom are plastics processed in the uncolored
state into a final product. Aesthetic perspectives
and requirements must be taken into account for
consumer products or visible components in the
automotive sector, for example. However,
functional colorations, such as signal colors, are
also required in certain areas.
most important representative is carbon black,
which besides being financially attractive also has a
positive impact on the properties of the plastic.
Carbon black already provides sufficient coverage
at very low concentrations from 0.5 to 1%. It is ideal
for coloring the laser-absorbing joining partner, as it
is absorbed in nearly the entire wavelength range.
For this reason, colorants are added to the plastics
during processing, which change the outer
appearance according to the specifications.
Creating a color matrix of laser-transparent and
laser radiation absorbing color settings is not
possible due to the large number of variables, such
as paint, plastic, section thickness, laser
wavelength, speed of processes, approvals, etc.
Therefore, a degree of difficulty of the color
combinations has been created (see chart) with
which an initial indication for the use of color can be
made for laser plastic welding.
An equally significant group among the inorganic
color pigments is white pigments. The most
significant representatives today include titanium
dioxide TiO2, zinc white ZnO and zinc sulfide ZnS.
Since white pigments are highly reflective and
scattering in response to the radiation used in laser
plastic welding in the near infrared range, very
white-colored components are very difficult to weld.
In contrast to the colorants, the pigments in the
plastic are almost insoluble. The pigments can be
inorganic or organic in nature, whereby the
inorganic pigments are completely insoluble.
Due to the specific requirements of laser radiation
welding for coloration regarding transmission and
absorption for wavelengths in the near infrared
range, special additives and colorings have been
developed by the plastics manufacturing industry
and colorant producers that meet the set
requirements. The now likely most well-known
coloring, which was specially developed for the
laser welding of plastics, is the IR-transparent black.
Superimposing different, conditionally lasertransparent pigments creates an opaque, black
color impression of a component in the range of
visible light. Unlike other black colors, the coloring
thus produced is transparent to wavelengths in the
near infrared range.
The most important group among the inorganic
pigments is made up of the black pigments. The
The use of carbon black is not possible for color
settings that are bright and brilliant. However, nearly
Color additives can be distinguished between
pigments insoluble in plastic and soluble colorants.
Both groups have a strong influence on the laser
radiation welding of plastics, since they affect the
visual properties of thermoplastics in the near
infrared range.
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all colors can be set with the use of alternative NIR
absorbers.
Transparent combinations are possible by means of
a series of transparent NIR absorbers. Another
possibility for the joining of transparent components
is the use of laser wavelengths between 1.6 to 2
microns. In this range, absorption takes place
through the plastic itself so the weld can be done
without the addition of additives
Specifically, consultation with an EVOSYS
application expert will provide quick information.
Figure: Beam shadow of laser beam
5. Design layout of the joining zone
An essential component of a robust process is the
design of the joining zone. Basically, one can
distinguish between process-independent and
process-related criteria.
5.1 Process variant-independent criteria
The following criteria and statements are generally
applicable to all process variants.
Clamping device bearing surface
An essential component of the laser welding
process is the joining force between the two joining
partners. It is necessary to use a clamping device in
order to introduce this force into the components to
be joined.
Material thickness
Depending on the material used for the lasertransparent joining partner, the material thickness in
the area where the laser radiation passes through
the transparent component should not be designed
to be excessively thick. A good rule has been
proven that the material thickness is chosen
similarly to the planned width of the weld seam.
However, as a general rule of thumb: the thinner the
better! Due to the variety of different materials and
color and filler variants, it is always recommended
that you consult with an experienced specialist from
the company EVOSYS as well as perform a
transmission measurement to assess the
weldability.
Accessibility for the laser beam
Due to the characteristic beam shape for the
typically focused laser beam, it is necessary to
ensure a shadow-free accessibility in the area
above the joining zone.
Figure: Application of clamping force
Typically an application of force is required in the
immediate vicinity of the entire weld seam. To
support the corresponding clamping device,
sufficient space must be provided in the area of the
joining zone and accessibility must be ensured for
the laser beam. The force used usually is thereby in
a range of 2-5 N / mm^2 with respect to the weld
seam area.
Also a corresponding counter surface must
provided for receiving the applied force. In
majority of cases, this is done directly via
component and a component jig, which follows
shape contour of the overall assembly.
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5.2 Process variant-specific criteria
The following criteria are especially relevant for the
process variants mentioned.
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Contour welding - tolerances
Since no relative movement of the joining partners
to each other can take place in a contour welding
process, but thermal contact between the two
joining partners is also necessary, the requirements
for the molded part tolerances are higher than for
the other process variants.
As an example of the magnitude of the required
tolerances, the flatness tolerance of a joining part
with a flat weld geometry and lateral dimensions in
the range of 100 mm x 100 mm should be
mentioned at this point. For such an assembly, a
flatness tolerance of no more than 0.08 mm would
be applied.
Minor differences can be compensated for by higher
clamping forces, but these lead to stresses within
the assembly after the welding process.
The application engineers from EVOSYS are happy
to help you with the proper choice of tolerances for
your application.
Quasi-simultaneous - collapse
It is important in this context to take into account the
relative movement and the contraction during the
cooling phase
Usually the collapse for components with
dimensions in the range 100 mm x 100 mm is about
0.1-0.5 mm.
Specifically, consultation with an EVOSYS
application expert will provide quick information.
Quasi-simultaneous melt flash space and melt
coverage
For simultaneous and quasi-simultaneous welding,
melt material is displaced from the joining zone in
the process due to the afore-described melting of a
collapse. It is important to provide sufficient space
for the displaced melted mass in the adjoining
regions of the joining zone.
Since this melted mass usually interferes
aesthetically or - in the case of glass-fiber reinforced
materials - poses a risk of injury during subsequent
manual processing, it is recommended to provide
an additional structural cover.
Unlike with a contour welding process, a relative
movement of the assemblies to each other occurs
during simultaneous and quasi-simultaneous
welding. Usually a so-called weld rib is provided on
the laser-absorbent molded part, which is melted off
during processing. The laser-transparent molded
part moves towards the laser-absorbent part. If the
laser is switched off at the end of the process, this
is followed by a cooling phase in which the cooling
causes a shrinkage of the material.
Figure: Melt flash cover
Specifically, consultation with an EVOSYS
application expert will provide quick information.
Figure: collapse
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Checklist
6. Checklist
Assembly Part
Selected process variant?
Is the material selection of the base material weld able?
Laser-absorbing molded part pigmented in an absorptive manner?
Laser-transparent molded part colored in a laser-transparent
manner?
Accessibility for the laser beam ensured?
Bearing surface of clamping technology available?
Direction of contact pressure of clamping technology correctly
designed?
Receiving of lower component half possible?
Positioning of the component halves to each other ensured?
Collapse considered, if necessary?
Flow space available for melt flash?
Cover for melt flash available?
Note
All information about the laser beam plastic welding process mentioned in this document is to be
understood as a rough overview of the requirements of the process. Due to the many different
influencing factors, we recommend including the specialists from Evosys Laser GmbH at the earliest
possible stage. All information in this document is not binding and does not claim to be exhaustive.
This document and the entire contents of the document as a whole or in part are protected by
copyright. The reproduction, translation or duplication of the content as a photocopy or in any digital
form is permitted only with written permission of Evosys Laser GmbH.
Creation Date
December 2016
Copyright
© 2016 Evosys Laser GmbH
Evosys Laser GmbH
Henri-Dunant-Straße 8, 91058 Erlangen
Tel. +49 9131-907138-0
[email protected], www.evosys-laser.com
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